A possible mechanism for the effect of solar variability on climate, whereby solar variability acts over the stratospheric pressure system transmitting the changes top-down, and over ocean temperatures bottom-up.
Summary: In part A, we established the existence of a ~ 2400-year climate cycle, discovered in 1968 by Roger Bray. In part B, we confirmed Bray hypothesis that the climate cycle correlates with a ~ 2400-year cycle in the production of cosmogenic isotopes, that corresponds with clusters of solar grand minima at times of abrupt cooling and climate deterioration. Now we discuss a possible mechanism for the effect of solar variability on climate. Proxy evidence, instrumental era measurements, and reanalysis, suggests that solar variability acts over the stratospheric pressure system transmitting the changes top-down, and over ocean temperatures bottom-up. Low solar activity appears to induce a contraction of the Hadley cells, and an expansion of the polar cells, steepening the Equator-to-Pole temperature gradient, decreasing global temperatures and changing wind and precipitation patterns. A persistent North Atlantic oscillation negative phase intensifies the effects over this particularly sensitive region.
Solar variability effect on climate
In part B, we answered the objection that the cosmogenic record is likely to be contaminated by climate and therefore is more of a climatic record than a solar activity record. The available evidence rules out that objection showing that the cosmogenic record, when adequately corrected for geomagnetic and climatic effects, reflects mainly solar activity.
The second objection that opponents of the solar-climate theory raise is that there is no known mechanism by which small changes in TSI could cause an important effect on climate. This is actually a non-sequitur fallacy, because it assumes that the climatic effect must be due to changes in TSI when there is no evidence of it. The effects of the climatic Bray cycle shown by the reviewed proxies provide ample evidence of the mechanism involved, that is confirmed by instrumental measurements, reanalysis data, and climate modeling (reviewed by Gray et al., 2010).
Solar variability is higher at the short-wave part of the spectrum, as UV can change during the 11-year solar cycle by as much as 100%. Even though it constitutes a small part of TSI, UV radiation has specific effects in the stratosphere and the oceans. UV radiation of different wavelengths at different heights both creates and destroys ozone in the stratosphere, at the same time warming it. The changes in ozone are difficult to track, because they are affected by ozone transport within the stratosphere and to the troposphere, and by chemical processes that destroy ozone from volcanic eruptions and anthropogenic emissions, but the measured changes in total ozone during the solar cycle are on the order of ~ 3% (figure 65 A). This is 30 times more variation than for TSI during the 11-year solar cycle. As the stratosphere has a very low density, the changes in ozone are accompanied by significant changes in its temperature profile that can be of 0.5-1 °K in the tropical stratosphere for the solar cycle, and by changes in pressure that alter the geopotential height of the tropopause (figure 65 B). As ozone is unequally distributed latitudinally these changes alter both the temperature and pressure gradients between the equatorial and polar stratosphere. The pressure changes are transmitted all the way down to the surface altering the tropospheric pressure distribution and strength. The process is affected both by seasonality and stratospheric oscillations, like the Quasi-Biennial Oscillation. A higher probability of winter blocking days over the North Atlantic during periods of low solar activity has been demonstrated by several authors. This has the effect of increasing the probability of very cold events, like the 2010 Northern Europe snow storm at the solar cycle 23-24 minimum.
Figure 65. Stratospheric effects of solar activity changes. A). Deseasonalized, area-weighted total ozone deviations from five datasets for the latitude bands 25°S- 25°N. The solar flux at 10.7 cm is shown in the upper graph as a proxy for solar variability. Source: M.P. Chipperfield et al., 2007. WMO Sci. Asses. Ozone Depl. 2006. B). Time series of the 10.7 cm solar flux (light gray, dashed line) and of the annual mean 30-hPa heights (medium gray, thin solid line) and their three-year running means (heavy solid line) in geopotential km for the gridpoint 30N/150W. Source: K. Labitzke 2001. Meteorol. Z. 10, 83-90.
The changes in the meridional pressure gradient induce a global atmospheric reorganization, but since after a few years the direction of the change reverts with the solar cycle, the effect of the 11-year solar cycle on the weather is negligible. However, when the level of average solar activity changes over a period of several decades, the atmospheric reorganization advances, becoming noticeable first and causing important changes in the climate later. One of the effects of the changes in the meridional pressure gradient is to cause changes in the meridional temperature gradient (also Equator-to-Pole Temperature Gradient, EPTG; Soon & Legates, 2013). The importance of this gradient cannot be overstated, as it acts as the thermodynamic engine of the planet’s climate, and its periodic changes with the Milankovitch obliquity cycle correlate with the glacial cycle and had been proposed as its causative agent (figure 16; Raymo & Nisancioglu, 2003). Soon and Legates (2013) have shown that the EPTG has been decreasing during periods of surface warming and increasing during periods of surface cooling (figure 66), and convincingly link them to changes in average solar activity.
Figure 66. The meridional temperature gradient. Annual-mean EPTG over the entire Northern Hemisphere (°C/°Latitude; thin line) and smoothed 10-year running mean (thick line) from 1850 to 2010. The values are expressed as anomalies from the average for the 1961-90 period. The scale is inverted, since the average EPTG is strongly negative, positive anomaly values reduce the gradient (warmer Pole and/or colder Equator) while negative anomaly values enhance it (warmer Equator and/or colder Pole). Periods of global warming result in a decrease of the EPTG. Source: W. Soon & D.R. Legates 2013. J. Atmos. Sol.-Terr. Phys. 93, 45-56.
It is known that at least since 1979 the Hadley circulation has been expanding poleward at a rate of 0.5-1 °Lat/decade in both hemispheres (Hu & Fu, 2007). The cause is uncertain and both ozone changes and GHGs have been proposed. Models indicate that the expansion could have been taking place for most of the 20th century. Sadourny (1994), using modeling, ascribed to solar activity decrease a contraction of the Hadley cell and associated monsoon systems as causing agents for the climatic changes that took place during the Maunder Minimum. The expansion of the Hadley circulation currently observed coincides with the decrease in the EPTG (figure 66) and is a logical explanation as it supposes an expansion of the tropics.
At the ocean surface UV radiation decreases to about 3-5% of TSI, but it can penetrate water as readily as the visible range and is more energetic, so a few meters into the oceans, UV radiation might be responsible for about 7-10% of the ocean warming produced by solar radiation and it can change a few percentage points during the solar cycle. A close correlation between subsurface water temperatures and TSI has been reported south of Iceland between 818-1780 AD (Moffa-Sánchez et al., 2014). SST at the North Atlantic has also been decreasing since 2006, coinciding with the decrease in average solar activity.
With all this information from the instrumental era, and the information reviewed from proxy records covering past lows of the Bray cycle, an attempt can be made to explain the effect of prolonged low solar activity on climate change.
When the time for a new low in the solar Bray cycle approaches, solar activity starts to decrease, but it does so mainly at the lows of the 208-year de Vries cycle that become more pronounced due to its modulation by the Bray cycle. The probability of a SGM increases and when it finally takes place it can be of the Spörer (~ 150 years) or Maunder (~ 80 years) types, with a higher tendency to produce a cluster of SGM spaced about 200 years, according to the de Vries cycle. Solar activity goes to minimum values at the SGM and the changes in the stratospheric ozone, temperature and geopotential height induce an atmospheric reorganization characterized by the weakening of the stratospheric polar vortex, the progressive expansion of the polar cells and the contraction of the Hadley cells, and as a result a steepening of the EPTG that increases the amount of heat lost by the planet. This reversible process of atmospheric reorganization is cumulative and proceeds very slowly. This explains why the 18th century, with a solar activity level similar to the 20th century had a different climate. The 20th century expansion of the Hadley cells and reduction of the EPTG were built upon the levels reached over the previous two centuries.
The contraction of the Hadley cells at the SGM explains the southward displacement (weakening) of the monsoons associated with the Hadley circulation (figure 54 f), and the decrease in wind strength at the Santa Barbara basin that increases precipitation (figure 54 e). The expansion of the polar cells explains the increase in wind strength over Iceland (figure 52 d), that appears to depend on Milankovitch forcing. With the expansion of the polar cells there is an increase in polar circulation driven by the strengthening of the Siberian high, that produces an increase in salt deposition over Greenland (figure 52 a & b).
In the North Atlantic the decrease in pressure differential causes the atmosphere to enter persistent NAO negative conditions (figure 52 e) as shown in figure 67. This causes the Icelandic low and the Azores high to be in a weak state more often, reducing the strength of both the Westerlies and storm tracks and causing them to move southward. Precipitation levels increased in Central (figure 54 a), and Southwestern Europe (figure 54 c). The weakening of the Westerlies reduces the contribution of fresh cold subpolar gyre waters to the NAC that becomes warmer and saltier (figure 53 b). The Jet stream pushes southward, cooling Northern Europe and Northeast North America, and warming Greenland unless very cold Arctic conditions dominate. The warming of the NAC (figure 53 d) increases precipitation over Ireland (figure 54 d) and Norway (figure 54 b), while colder winter conditions induce glacier expansion at a global scale (figure 51). The warmer saltier NAC prevents the shutdown of the NADW that experiences a decrease (figure 53 a), due to the AMOC reduction in response to weaker Westerlies. Glacier growth, colder conditions, and the advection of warmer waters to the Arctic, favor an increase in drift ice both North of Iceland (figure 53 c) and in the North Atlantic (figure 55 f).
Figure 67. Summary of the climatic effects associated to the lows of the Bray cycle. Global effects are mediated by the contraction of the Hadley cells and expansion of polar cells that steepen the meridional temperature gradient causing global cooling. The contraction also restricts monsoon patterns, causing drier conditions in sub-tropical latitudes. El Niño conditions become infrequent, altering precipitation patterns. The North Atlantic realm is pushed into persistent AO/NAO negative conditions that are characterized by weak Iceland low pressure and Azores high pressure centers. This decreases the strength of the Westerlies that take a more southern path changing precipitation patterns over Europe, and causing blocking conditions over the Atlantic that allow Arctic cold air to drift southward. The contribution of cold fresher subpolar gyre waters to the North Atlantic Current (NAC) decreases, becoming warmer and saltier, and increasing winter precipitation over northern Europe causing glacier advances. A strong Siberian high brings colder conditions over northern Eurasia and increases polar circulation over the Arctic and Greenland, increasing the amount of southward drift ice. Greenland experiences an inversion as masses of cold air are displaced towards northern Europe and North America. NADW labels North Atlantic deep-water currents. Black dots are the location of some of the proxies discussed in the text that display a clear ~ 2400-year periodicity.
The global effects of the atmospheric reorganization induced by prolonged low solar activity are thus multiplied because this condition pushes the North Atlantic atmosphere-oceanic system into a persistent NAO negative condition. The hydrological effects of the Hadley cells contraction are mainly zonal in both hemispheres. However, the cooling effect of the increased EPTG is global and propagates over nearly all the oceans (figures 55 c, d, e, & 56), resulting in global cooling (figure 55 a). The climatic effects of low solar activity over the North Atlantic realm are particularly intense (figure 67). This is the reason why scientists argue over the regional versus global extent of the MWP and the LIA. The North Atlantic is a hot spot for planetary climate variability during both the glacial (D-O cycle) and interglacial periods.
The solar-climate debate has been long but not fruitful so far. Astrophysicists and instrumental-era climatologists are entrenched in the low energy changes in TSI as an argument against the connection. But the stratosphere is very rarefied and little energy is needed to alter it significantly. Afterwards the climate system provides the rest of the energy by oscillating to a different state through internal variability. The EPTG is a crucial element that determines how much work the energy does on its way out of the planet (figure 68). For essentially the same solar output, as far as we know, the EPTG has determined if the planet is in an icehouse, as currently, or in a hothouse as during the Eocene. Finally, the North Atlantic Oscillation amplifies the climatic response, turning a slow change in solar output into the cold winters of the Maunder Minimum. The paleoclimatologists are correct in sticking to the evidence that solar variability, when prolonged, has a disproportionate effect on climate change, and in the end, they will win the debate. Astrophysicists are looking to the wrong solar system body to learn about the history of the sun. It is best recorded here on the Earth. The sun might not have a memory, but it is subject to multi-millennial cycles whose cause will have to be elucidated. Recognizing the existence of those cycles is an important first step.
Figure 68. Pole-to-pole temperature gradients for the planet. Pole-to-pole temperature curves representative of climatic conditions ranging from Extreme Hothouse to Severe Icehouse. The numbers along the right side indicate the corresponding global Mean Annual Temperature for each curve (modern MAT is 14.3°C). The number along the left side of the curve is the tropic-to-pole temperature gradient, or how quickly the surface temperature cools as you approach the pole. Polar temperatures for each of the seven pole-to-pole temperature curves are also listed (modern Antarctica is -50°C). The average temperature at the Equator, kept constant in the graph for convenience, has also changed through time, but a lot less than the rest of the planet. The curves for each hemisphere are independent. Current climate is described by curve 7 for the southern hemisphere and curve 6 for the northern hemisphere. We are now in Ice House conditions. Source: C.R. Scotese 2015. PALEOMAP Project.
5) The solar activity Bray cycle appears to act on climate both through changes in the stratospheric pressure system that are transmitted downwards to the troposphere causing an atmospheric reorganization, and through changes in the amount of energy warming the oceans.
6) Proxy evidence, instrumental era measurements, and reanalysis support the idea that lows in the Bray cycle and prolonged below average solar activity cause a contraction of the Hadley cells, and an expansion of the polar cells, steepening the Equator-to-Pole temperature gradient, decreasing global temperatures and changing wind and precipitation patterns.
7) In the North Atlantic region, in addition, the Arctic and North Atlantic oscillations enter a persistent negative phase during the lows of the Bray cycle, causing an intensification of winter climatic effects and making this region particularly sensitive to low solar activity. This explains why the Little Ice Age, while global, was particularly strong over Europe and North America.
I thank Andy May for reading the manuscript and improving its English.
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